Autoxidation of architectural coating composition takes place at ambient temperatures and in natural daylight and so occurs slowly unless it is accelerated by the presence of a both a surface promoter system and a through promoter system. Such promoters are often called “driers” but in this description they will be called “promoters” to avoid confusion with the more usual concept of drying which involves loss of solvent from the coating composition after it has been applied to a surface.
Architectural coating compositions such as paints, lacquers and varnishes which autoxidise at ambient temperatures in natural daylight are commonly used on site to coat surfaces found in or around buildings. Accordingly, the compositions are usually called “autoxidisable architectural coating compositions” and they need to be suitable for application at ambient temperatures in natural daylight by unsophisticated users having no respiratory protection and who use simple application tools such as brushes, rollers or pads. Once applied to a surface, the coating dries (in the sense of losing solvent by evaporation) and undergoes autoxidation promoted by various metal ions to form a solid coherent dried coat bonded to the surface.
Autoxidisable architectural coating compositions may comprise either autoxidisable binder polymer dissolved in organic solvent such as white spirit or dispersions of autoxidisable binder polymer in water. Generally, the compositions will also contain non-film-forming components such as particulate inorganic and/or organic pigments or opacifiers (for example particulate titanium dioxide, especially rutile or polymeric organic particles containing voids) or extenders (for example chalk, dolomite, clays or talc) as well as other optional ingredients such as matting agents (for example silica), structuring agents (for example titanium or zirconium chelates or laponite or bentonite clays), antifoaming agents and biocides. Pigment grade rutile particles are coated with other oxides such as silica in order to minimise degradation of the coating by light.
The autoxidisable film-forming binder polymer is an essential component of an autoxidisable architectural coating composition partly because it autoxidises to form the coherent coat which bonds to the surface to which it has been applied and partly because it binds together any non-film-forming components which may be present in the composition such as those described above. During autoxidation, autoxidisable moieties in the binder polymer are believed to form crosslinks between adjacent polymer chains resulting in a significant increase in the weight average molecular weight of the binder polymer.
The best known autoxidisable architectural coating compositions contain autoxidisable binder polymers which are alkyd resins. Alkyd resins are described on pages 211 to 218 and 228 and 229 of Volume 1 of the 2nd Edition of the book “Outlines of Paint Technology” by W M Morgans and published in 1988 by Griffin of London. The contents of these pages from Morgans are herein incorporated by reference. Morgans explains that, essentially, alkyd resins are condensates of dicarboxylic acids with polyhydric alcohols to which are attached long chain moieties containing autoxidisable ethylenic unsaturation. These long chain moieties are usually obtained from vegetable oils. Alkyd resins containing 60 to 85 wt % of the moieties are often called “long oil” alkyds whilst those containing 45 to 60 wt % are called “medium oil” alkyds and those with only 25 to 45 wt % are called “short oil” alkyds. Alkyd resins can be vulnerable to yellowing with age and so they should avoid the presence of cyclo-alkenyl moieties which aggravate the yellowing problem. Alkyd resins should also avoid oxidisable moieties such as allyl ether groups which degrade to give acrolein which is a potent and toxic lacrimator painful and possibly dangerous to users not having respiratory protection. Examples of autoxidisable moieties which are suitable include those derived from linseed oil, soya bean oil, safflower oil, cotton seed oil, dehydrogenated castor oil, tall oil and tung oil.
Examples of suitable dicarboxylic acids for use in making alkyd resins include ortho-phthalic, iso-phthalic, terephthalic, maleic, fumaric, adipic and sebacic acids or their anhydrides. Suitable polyhydric alcohols include ethylene glycol, glycerol, pentaerythritol, 1,2 propylene glycol, trimethylol propane and neopentyl glycol.
As mentioned above, unpromoted autoxidisable binder polymers such as alkyd resins autoxidise far too slowly at ambient temperatures in natural daylight to be of much practical use in architectural paints because they are applied on site as opposed to being applied in a factory where autoxidation can be conveniently accelerated by use of high energy irradiation (eg. actinic irradiation) or by stoving at temperatures well above ambient. Clearly, it is not very practicable to provide high energy irradiation or stoving facilities on site especially for use by unsophisticated users and so architectural coating compositions need to contain promoter systems to accelerate their autoxidation. Typical promoter systems are described (using the alternative name of “driers”) on pages 159 and 160 of the 3rd Edition of the book “Introduction to Paint Chemistry and Principles of Technology” by GPA Turner published in 1988 by Chapman and Hall of London. The contents of these pages 159 and 160 are herein incorporated by reference.
As indicated earlier, there are two types of promoter systems commonly employed in architectural paints, namely “surface promoters” and or “through promoters”. Surface promoters accelerate autoxidation in the surface layers of an applied coating probably by catalysing the uptake of oxygen and the decomposition of peroxides to form free radicals which cause crosslinking. In contrast, through promoters accelerate the increase in weight average molecular weight of the binder polymer in the lower levels of the coating. Conventional surface promoters comprise carboxylates, preferably octoates, 2-ethyl hexanoates or naphthenates of cobalt, manganese, vanadium, iron, chromium, copper, tin and cerium. Conventional through promoters comprise compounds of the above carboxylates with one or more carboxylates of zirconium, calcium, barium, strontium, lithium, sodium, potassium, zinc, neodymium, bismuth, lead and aluminium as well as alkoxides of aluminium.
To achieve sufficiently fast rates of surface autoxidation, conventional surface promoters are normally used in amounts such that the concentration of their transition metal ions based on the weight of all the autoxidisable binder polymer in the composition is at least 0.001 wt % up to 0.1 wt % with the preferred range being 0.04 to 0.07 wt %. To achieve sufficiently fast rates of presumed autoxidation in the lower levels of a coating, conventional through promoters are normally used in amounts such that the concentration of all their metal ions based on the weight of the autoxidisable binder polymer in the composition is at least 0.3 wt % and generally up to 2 wt % with the preferred range being 0.5 to 1.5 wt %.
Several transition metal ions, notably those of copper, iron, chromium or manganese discolour coating compositions unless the composition is heavily pigmented. Discoloration due to cobalt ions is much less and so cobalt carboxylates have established themselves as the surface promoters ubiquitously used in architectural paints because their use means that it is easier to make the popular light pastel shades of colour. However there are now rumours that cobalt ions may be carcenogenic, and so they should be used only in systems where the concentration of cobalt ions is below 0.01 wt % based on the autoxidisable binder copolymer. Preferably the promoter systems should contain no cobalt ions at all, that is to say they should be non-cobalt promoter systems.
Photoinitiators are often used together with high energy irradiations (often called “actinic” radiation) to accelerate the crosslinking of paints applied under factory conditions. Pages 222 and 223 of Turner, ibid, describe industrial coating compositions containing photoinitiators and either unsaturated polyesters or unsaturated acrylic (including methacrylate) polymers. Turner states that flat articles coated with the, compositions may be conveyed under powerful ultraviolet lamps which irradiate the coated articles with high energy artificial ultraviolet light which decomposes the photoinitiator generating free radicals which harden the coating and create a coherent dried coat. Similarly, United States Patent Specification U.S. Pat. No. 4,387,190 (published in 1983) discloses that a combination of photoiniators and actinic radiation can be used as an alternative to autoxidation, to polymerise coatings containing dicyclopentenyl methacrylates or alkoxy methacrylates. Actinic radiation is high energy radiation of the type conveniently available under factory conditions whereas only daylight or low energy lighting is realistically available on site in a building. This means that those coatings of U.S. Pat. No. 4,38,190 which contain photoinitiators are not architectural coating compositions.
European Patent Specification EP 0 234 641A (published in 1986) discloses autoxidisable coating compositions which contain allyl ether groups in order to permit the use of less volatile solvents. As mentioned above, allyl ether moieties degrade to give acrolein which is a potent lacrimator and so the coating compositions are not suitable for use as architectural coating compositions because architectural coating compositions must be usable by persons not equipped with respirators.
It is an object of this invention to provide an autoxidisable architectural coating composition suitable for application to surfaces at ambient temperatures and natural daylight by unsophisticated users having no respiratory protection which composition contains an autoxidisable binder polymer and promoter system containing low to zero concentrations of cobalt ions for promoting autoxidation of the binder polymer